One resource is sellyourmac.com. They'll give you an instant quote, which is good info even if you don't plan on selling to them. (They do almost all Apple hardware)
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Nathan GreensteinNov 12 '11 at 18:11

For the most recent price for a particular model in some particular condition, I try to find a bunch of completed sales on eBay with best matching descriptions and pick the median closing bid price.

Noting the variance in prices is also important, since used devices don't really have any fixed price. It depends on the buyer, seller, location and date (phase-of-the-moon, last poker hand, etc.). YMMV.